Magnetoresistive sensors are likely to be important components in automobiles of the 1990's and beyond. These sensors can function as position-sensitive or speed-sensitive devices and will provide the feedback necessary for sophisticated computer-control of engine, transmission and braking functions. The sensors operate by monitoring the resistance of a magnetoresistive element in proximity to a magnet; the resistance of the element depends strongly on the strength and direction of the impinging magnetic field, as well as on the electronic mobility of the element itself. Since the III-V semiconductor indium antimonide (InSb) has the highest carrier mobility of any known semiconductor (77,000 cm.sup.2 /V.sec for electrons), it is one of the strongest candidates for use as the magnetoresistive element of such sensors.
Magnetoresistive sensors require a relatively thin magnetoresistive element, from 15 .mu.m to 20 nm depending on the specific properties of the film material. Electrodeposition of InSb is potentially a convenient method of obtaining such films since the deposition of any desired film thickness can be easily accomplished by controlling the current or the deposition time or both.
Low-temperature molten salts comprised of aluminum trichloride (AlCl.sub.3) and an organic chloride such as 1-butylpyridinium chloride constitute a well-studied class of electrolytes. The reaction by which the melts are formed can be written as EQU RCl+AlCl.sub.3 .rarw.---.fwdarw. R.sup.+ +AlCl.sub.4.sup.- (1)
where RCl is the organic chloride. A wide range of acidities is available in these organic chloroaluminate melts by varying the molar ratio of AlCl.sub.3 to organic chloride. Melts with molar ratios less than unity are basic due to the chloride ions from the dissociation of excess organic chloride. For ratios greater than one (excess AlCl.sub.3), the corresponding melts have significant Lewis acidity.
Low-temperature melts can be made by using GaCl.sub.3, another Group III metal chloride, in place of AlCl.sub.3. U.S. Pat. No. 4,883,567, the disclosure of which is hereby incorporated by reference, is directed to the room-temperature electrodeposition of GaAs from such an organic chlorogallate melt containing GaCl.sub.3 and 1-methyl-3-ethylimidazolium chloride (ImCl).
While it is likely that InSb can be electrodeposited from either an organic chloroaluminate or chlorogallate molten salt, contamination of the deposit with Al or Ga, respectively, would be likely to occur. Therefore, a melt containing In ions and Sb ions as the only metallic ions would be preferred for InSb electrodeposition.
The electrolyte used in the present invention, the InCl.sub.3 /1-methyl-3-ethylimidazolium chloride (InCl.sub.3 /ImCl) molten salt, is a novel material; it has never been prepared before and therefore has never been used as an electrolyte for electrodeposition of any kind. Unlike the GaCl.sub.3 /ImCl melt which is liquid at room temperature over a range of composition ratios, the InCl.sub.3 /ImCl melt is liquid at room temperature only at composition ratios very near 45:55 (molar ratio of InCl.sub.3 to ImCl)----and even then it is very viscous. Furthermore, if one attempts to prepare the InCl.sub.3 /ImCl melt exactly as the GaCl.sub.3 /ImCl melt was prepared in U.S. Pat. No. 4,883,567, by mixing the two solid reactants at room temperature, a sticky mass is obtained rather than a clear liquid. Thus, the reaction of InCl.sub.3 /ImCl with SbCl.sub.3 is not advanced.
While the chemistries of the Group III elements share some similarities, there are also marked differences such that one cannot reliably predict the effect of substituting one element for another in many chemical situations. For example, while Al cannot be electrodeposited from the basic AlCl.sub.3 /ImCl melt (excess of ImCl), Ga can be deposited from the analogous basic GaCl.sub.3 /ImCl melt.
One well-known trend in the chemistry of Group III elements is the increasing importance of the univalent oxidation state versus the trivalent oxidation state as the atomic weight increases. Thus the importance of the univalent state is expected to be greater in the chemistry of the heavier In than in the corresponding chemistries of either Al or Ga.
The effects of this relative importance of the In(I) state on the electrodeposition mechanism are not predictable. Thus one might have predicted that In deposition from a melt (such as InCl.sub.3 /ImCl) would be impossible since the reduction of In(III) might proceed to a stable In(I) species which could not be further reduced (i.e., the organic portion of the melt would be reduced first).
The prior art suggest other notable differences over the present invention which cast doubt on any reasonable expectation of success in electrodeposition of InSb from an InCl.sub.3 /ImCl and SbCl.sub.3 material. Wilkes et al. "Dialkylimidazolium Chloroaluminate Melts:A New Class of Room-Temperature Ionic Liquids for Electrochemistry, Spectroscopy, and Synthesis," Inorg. Chem., 21 (1982) 1263, and Wicelinski et al., "Low Temperature Chlorogallate Molten Salt Systems," J. Electrochemical Society, 134 (1987) 262, disclose that both AlCl.sub.3 /ImCl and GaCl.sub.3 /ImCl melts are liquids at room temperature for a relatively wide mole percent of AlCl.sub.3 and GaCl.sub.3, respectively. Notwithstanding, Wilkes et al., discloses that Al cannot be deposited from such a basic melt (unlike Ga deposition). Lipsztajn et al., "Increased Electrochemical Window in Ambient Temperature Neutral Ionic Liquids," J. Electrochemical Society, 130 (1983) 1968, and Lai, et al. "Electrodeposition of Aluminum in Aluminum Chloride/1-Methy-3-Ethylimidazolium Chloride," J. Electro and I. Chem. 248 (1988) 431, disclose that the reduction leading to Al deposition is by the Al(III) dimer. Verbrugge et al., "Microelectrode Study of Gallium Deposition from Chlorogallate Melts," AICHE Journal 36 (1990) 1097, disclose that the reduction leading to Ga deposition is by the Ga(III) dimer. Cotton et al., "Advanced Inorganic Chemistry," 5th Ed. (1988) discloses: 1) the propensity of Group IIIA elements to exist in the (I) state increases as the group is descended from Al to Tl; 2) Indium dimers containing In(III) are typically unstable in nonaqueous solvents; and 3) Tl(I) dominates thallium chemistry, rather than Tl(III). Thus, the prior art cast considerable doubt that the In(III) dimer would exist analogous to (al.sub.2 Cl.sub.7).sup.31 and (Ga.sub.2 Cl.sub.7).sup.-, the species that lead to Al and Ga deposition, respectively.